1. Field of the Invention
This invention generally relates to casing collar locator (CCL) tools and, more specifically, to circuits and methods for selectively operating a CCL tool in passive or active mode.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Wells are drilled for a variety of reasons, including the extraction of a natural resource such as ground water, brine, natural gas, or petroleum, for the injection of a fluid to a subsurface reservoir or for subsurface evaluations. Before it can be employed for its intended use, a well must be prepared for its objective after it has been drilled. The preparation is generally referred to in the industry as the “well completion phase,” and may include casing the borehole of the well to prevent collapse, as well as other processes specific to the objective of the well and/or the geomechanical properties of the rock in which the well is formed. For example, typical well completion processes for oil and gas wells may include perforating, hydraulic fracturing and/or acidizing.
After a borehole is drilled, the well may be cased by inserting sections of metal pipe or “casing strings” into the borehole, and connecting end segments or “joints” of the casing strings together to form a well casing. Each section of metal pipe or “casing string” may generally have the same length. In some cases, the joints may be threaded and coupled together using metal couplings called “collars” or “casing collars.” Once a casing collar has been threaded onto an end section of pipe, another section of pipe may be inserted into the borehole and threadably coupled to the casing collar.
In this manner, a cased well may include a plurality of stacked casing strings, which are coupled together at each end segment or joint by a casing collar. Inclusion of the casing collar increases the thickness of the well casing at the location of the casing collar joint. In some cases, an air gap may also exist at the casing collar joint between the ends of adjacent casing strings. As described in more detail below, the increase in metal thickness and/or the air gap at the casing collar joint may produce a magnetic field disturbance, which may be detected by a casing collar locator (CCL) tool.
In some cases, a cased well may be perforated to establish a flow path to formations outside of the wellbore. In the oil and gas industry, for example, a hole or perforation may be punched in the well casing to connect the well to a reservoir. In perforation jobs, a perforation gun (i.e., a string of explosive charges) is lowered into the borehole down to a desired depth and fired to perforate the well casing at that depth. Perforation guns are commonly run on wirelines (i.e., a steel cable encompassing power and data lines), so that electrical signals from the surface can be used to fire the guns.
Well logging, also known as borehole logging or wireline logging, is the process of generating a detailed record (referred to as a “well log”) of the geologic formations penetrated by the borehole. Some types of well logs can be done during any phase of the well, including drilling, completing, producing or abandoning. The oil and gas industry uses wireline logging to obtain a continuous record of the geomechanical properties of the rock in which the well is formed as a function of borehole depth. Wireline logging is generally performed by lowering a logging tool (i.e., a string of one or more borehole instruments) on the end of a wireline into a borehole and recording measurements obtained from a variety of instruments. The measurement data can be recorded either at the surface (real-time mode) or in the borehole (memory mode). Real-time data is recorded directly against measured borehole depth. On the other hand, memory data is recorded and depth data is measured simultaneously against time, and the two data sets are merged using the common time base to create an instrument response vs. depth log. Wireline logging may be performed before the well is cased to generate “open hole logs,” or after the well is cased to generate “cased-hole logs.”
Casing collar locating (CCL) tools are widely used for depth control and to correlate tool string positioning with other depth based logs during cased-hole logging and perforation operations. A CCL tool is an electromagnetic measurement device, which may be attached to a downhole tool and lowered into a borehole for detecting the presence of casing collar joints or other magnetic anomalies. As the CCL tool is lowered into the borehole, the borehole depth can be calculated by counting the number of casing collar joints detected by the CCL tool and summing the individual lengths of casing strings located between each detected joint. By correlating this calculated depth against a depth control log, which precisely shows the depth of each casing collar joint, the location of the downhole tool can be accurately determined. This enables the measurement data in cased-hole logs to be accurately correlated to borehole depth, and also enables precise positioning of perforation guns.
There are two different basic types of CCL tools: passive CCL tools and active CCL tools. The basic components of each type of CCL tool are the same and generally include at least one winding or coil, which is arranged next to at least one magnet (e.g., a permanent magnet or an electromagnet). When an active or passive CCL tool is lowered through a cased well, the magnetic field surrounding the CCL tool is disturbed when a change occurs in the effective magnetic permeability of its surroundings (e.g., in the presence of a casing collar joint, an air gap between casing strings or other anomalous features of the pipe). The change in magnetic field causes electrical pulses to be generated in the coil. In passive CCL tools, the coil is directly connected to the wireline and the electrical pulses are detected and recorded by equipment coupled to the wireline at the surface. In active CCL tools, an electrical circuit is coupled between the coil output and the wireline for amplifying the electrical pulses, and then either: coupling the analog signal to the wireline, or digitizing the signal and transmitting the digitized signal up the wireline. In active CCL tools, power is required from surface equipment to power the electrical circuit processing the detected signals.
Passive and active CCL tools have different advantages and limitations, and thus, are generally preferred for different types of borehole operations. Since passive CCL tools do not require power, they are typically preferred for use during perforation jobs because of safety concerns with inadvertent detonation of the explosive charges. On the other hand, active CCL tools are typically preferred for use during logging operations, since they usually have little to no DC current load (i.e., draw less current), and therefore, won't saturate the coil at higher logging speeds. Due to the costs associated with purchasing, maintaining and storing different types of CCL tools, it would be advantageous to provide a single CCL tool that can be selectively configured to operate in both passive and active modes.